Consolidated Balance Sheets Parenthetical (USD $) In Thousands, except Share data	Sep. 30, 2011	Dec. 31, 2010
Allowance for doubtful accounts	$ 5,325	$ 4,925
Accumulated depreciation	$ 789,375	$ 731,453
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	4,500,000	4,500,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	48,432,860	48,408,259
Common stock, shares outstanding	48,432,860	48,408,259